Active/passive monolithically integrated channel filtering polarization splitter

ABSTRACT

An InP on-chip polarization splitter is proposed base on an arrayed waveguide grating that is composed of waveguides having birefringence. The present invention also performs channel filtering and demultiplexing that is useful in WDM/DWDM applications. The required waveguide structure is formed using active-passive monolithic integration platforms, providing the integration of the polarization splitter with active devices as well as passive devices.

FIELD OF THE INVENTION

This invention relates to the field of polarization splitters and, morespecifically, to active-passive integrated polarization splitters.

BACKGROUND OF THE INVENTION

On-chip polarization splitters (PS) are essential in optical circuitssuch as advanced photonic integration circuits (PICs). Such polarizationsplitters are indispensable for many applications that need polarizationdiversity or other polarization manipulations. In terms of materialsystems, on-chip PS has been demonstrated in LiNbO3, polymer, glass,III–V semiconductors and other passive materials. Among these materialsystems, only III–V semiconductors are naturally suitable foractive-passive monolithically integrated (APMI) applications. Thevarious methods for making on-chip PS may be characterized asdirectional coupler or waveguide crossing based, asymmetric Y-branchbased, Mach-Zehnder interferometer based, resonant tunneling based,multi-mode interference (MMI) based, and grating based.

In directional coupler or waveguide crossing based schemes, relativelylarge birefringence is used to make a directional coupler or waveguidecrossing in bar state for one polarization and cross state for another.Asymmetric Y-branch based PS need asymmetric birefringence in twodifferent waveguides and use mode evolution to ‘sort’ differentpolarizations into different waveguides. Mach-Zehnder interferometerbased PS make inputs of different polarizations experience differentoptical path length difference in the interferometer so that they go todifferent output waveguides. Resonant tunneling based PS introduce athird waveguide in the middle of a directional coupler so that only onepolarization is able to couple between two waveguides through tunnelingof a middle one. MMI based PS terminate MMI coupler at imperfect imagingplanes so that different polarization is able to couple to differentoutput waveguides. Grating based PS take advantage of the fact thatinput of different polarizations will be diffracted to different spatialpositions such that they may be separated.

However, such polarization splitters are not suitable for active-passivemonolithic integration. They either rely on large material intrinsicbirefringence which InGaAsP/InP material systems (i.e., for activefunction) do not possess, or they rely on air or metal claddingwaveguides for larger birefringence, which are not compatible with lowloss buried passive waveguides that can be integrated with activestructures. Ultimately, it is preferred to have active functions, suchas lasers, amplifiers, modulators, detectors and the like,monolithically integrated on a single chip with passive functions suchas wavelength multiplexing/demultiplexing, polarization control, andsignal filtering.

SUMMARY OF THE INVENTION

The present invention solves the deficiencies of the prior art byproviding an on-chip, active-passive monolithically integrated InPpolarization splitter based on AWG and waveguide birefringence andhaving multi-channel operation.

In one embodiment of the present invention an integrated polarizationsplitter includes an arrayed waveguide grating (AWG) having at least aninput coupler, an output coupler, and a plurality of waveguides ofunequal length connecting the input and output couplers. In theintegrated polarization splitter of the present invention at least twooutput ports of the AWG are positioned relative to an input port suchthat a first polarization component and a second polarization componentof a single channel input signal arriving at different phase fronts of afree space region at an output side of the AWG are respectively receivedby separate ones of the output ports such that the first polarizationcomponent and the second polarization component are split by the AWG.Furthermore, the polarization splitter is integrated usingactive/passive monolithic integration techniques such that thepolarization splitter is capable of being integrated with active devicesas well as passive devices.

BRIEF DESCRIPTION OF THE DRAWINGS

The teachings of the present invention can be readily understood byconsidering the following detailed description in conjunction with theaccompanying drawings, in which:

FIG. 1 depicts a high level block diagram of an embodiment of anintegrated channel-filtering polarization splitter of the presentinvention;

FIG. 2 depicts a high level block diagram of an experimental setup forthe polarization splitter of FIG. 1; and

FIG. 3 graphically depicts the output power of the TM (No.1) output andthe TE (No.5) output of the polarization splitter of FIG. 1.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures.

DETAILED DESCRIPTION OF THE INVENTION

Although various embodiments of the present invention are being depictedherein with respect to a single input polarization splitter splitting aninput channel into TE and TM modes, the specific embodiments of thepresent invention should not be treated as limiting the scope of theinvention. It will be appreciated by one skilled in the art informed bythe teachings of the present invention that the concepts of the presentinvention may be applied to polarization splitters having substantiallyany number of inputs and outputs for splitting an input channel intovarious polarization modes.

As a TE-mode wave and a TM-mode wave travel along a waveguide array inan arrayed waveguide grating (AWG), the waves arrive at different phasefronts before entering the free space (FS) region at the output side ofthe AWG. For an input of only one wavelength channel, the FS region thenfocuses the TE-mode wave and the TM-mode wave onto different image spotson the output side of the AWG, shifted by a birefringence shift inwavelength on the output focal plane. Therefore by positioning outputwaveguides at these image positions, the AWG splits the TE-mode wave andthe TM-mode wave for the single channel input. The positioning of theinput and output waveguides to achieve the splitting of the TE-mode waveand the TM-mode wave of a single channel input signal is discussedgenerally in Arjen R. Vellekoop, “A Small-Size Polarization SplitterBased On A Planar Optical Phased Array”, Journal of LightwaveTechnology, Vol. 8., No. 1., January 1990, which is herein incorporatedby reference in its entirety. Briefly stated, since the phase transferof an AWG is determined by the product of the propagation constant andthe total length of each channel, and because the propagation constantin a waveguide depends on the polarization as well as the wavelength ofa channel, the phased array may operate as a polarization splitter aswell as a wavelength multiplexer/demultiplexer. In addition, thepolarization splitter of the present invention may be used to split thepolarization of an input channel into other than just the TE-mode andthe TM-mode depending on the positioning of the output ports.

Furthermore, by virtue of the grating free spectral range (FSR), the AWGis also capable of polarization splitting wavelengths that are separatedby integer multiples of the FSR. Due to the effect of the AWG on apropagating optical signal, an input signal also gets filtered. Whilethis makes a splitter based on an AWG wavelength dependent, such asplitter may be desirable in certain applications, for example, becausesuch a splitter limits the impact of amplified spontaneous emission(ASE) noise.

FIG. 1 depicts a high level block diagram of an embodiment of anintegrated channel-filtering polarization splitter of the presentinvention. The polarization splitter 100 of FIG. 1 comprises an AWGcomprising an input waveguide 110, an input coupler (illustratively astar coupler) 120, an output coupler (illustratively a star coupler)130, a plurality of waveguides of unequal lengths (waveguide array) 140connecting the input coupler 120 and the output coupler 130, and aplurality of output waveguides 150. The AWG design of the polarizationsplitter 100 of FIG. 1 is a wavelength wrapping AWG design that has aFSR of 700 GHz and seven outputs separated by one channel spacing. Thechannel spacing is 100 GHz and the passband is designed to be Gaussianwith full-width-half-maxium (FWHM) of 30 GHz. Although in thepolarization splitter 100 of FIG. 1 the couplers are illustratively starcouplers, other coupler having substantially similar functions as thestar couplers, such as slab waveguide lenses, may be substituted for thestar couplers of the present invention.

The polarization splitter 100 of FIG. 1, is approximately 5 mm×6 mm insize. Typically the difference between the TE and TM effective index ofthe polarization splitter of the present invention, such as thepolarization splitter 100 of FIG. 1, is approximately 0.154%. Around thecenter wavelength of 1550 nm, this effective index difference causes abirefringence shift of substantially 2.4 nm, which, in the polarizationsplitter 100 of FIG. 1, equals approximately three channel spacings.

The polarization splitter of the present invention, such as thepolarization splitter 100 of FIG. 1, is integrated using active-passivemonolithic integration (APMI) techniques. For example, according to onefabrication technique, a shallow etched burried rib structure is usedfor forming the passive waveguides of the polarization splitter of thepresent invention. Such a technique provides record low propagation lossin InP material systems. An active section is then formed by anotherthin layer of multi-quantum-wells (MQW) directly on top of the rib whichis burried by the same re-growth that forms passive waveguides. As such,the polarization splitter of the present invention may be used as anactive device and a passive device. For example, the amplitude of the TEand TM modes may be independently controlled via, for example,amplification in the active sections, and as such an on-chip tunablepolarization controller may be achieved. Preferably, InP/InGaAsP is thematerial of choice for a polarization splitter in accordance with thepresent invention because this material allows for monolithicintegration with active photonic components such as transmitters,receivers, optical amplifiers, switches and the like. In this form,multiple waveguides may be placed in close proximity to each other suchthat each waveguide may be optimized for a specific optical function(e.g. active waveguide optimized for gain, passive waveguide optimizedfor ease of coupling, passive waveguide optimized for splitting,directional coupling or other passive devices).

Even further, a polarization splitter in accordance with the presentinvention is capable of active-passive monolithic integration (APMI)using other techniques known in the art. For example, a polarizationsplitter of the present invention may comprise opto-electronicintegrated waveguide devices utilizing a tilted valence band quantumwell semiconductor double heterostructure with one growth of the samewaveguide material. As such, a polarization splitter of the presentinvention may be operated with no bias for normal passive operation orwith reverse bias for operating as an active device. Such a technique isdiscussed generally in U.S. Pat. No. 5,953,479, issued to Zhou et al. onSep. 14, 1999, which is herein incorporated by reference in itsentirety.

Referring back to FIG. 1, in the polarization splitter 100, outputwaveguides No.1 and No.5 are illustratively depicted as the TM outputand the TE output, respectively, for a wavelength channel centeredaround 1550 nm. As in typical AWG configurations, the numbering of theoutput waveguides of the polarization splitter 100 starts from the innerside (shorter waveguide side) to the outer side (longer waveguides side)of the curved waveguide array. The slightly higher effective index ofthe TE-mode would make the TE-mode appear about three channels to theinner side of the TM output. Given the fact that the output waveguideNo.1 is matched by the output waveguide No.5 and that the birefringenceshift is close to three channel spacings, it follows that the waveguideNo.1 is the TM output and the waveguide No.5 is the TE output for the1550 nm channel and channels that are integer numbers of the FSR awayfrom 1550 nm.

In a polarization splitter of the present invention, such as thepolarization splitter 100 of FIG. 1, a single channel signal enters theinput waveguide 110 and is coupled into the AWG. As the TE-mode wave andTM-mode wave of the single channel input signal travel along thewaveguide array 140, they arrive at different phase fronts beforeentering the free space region of the output coupler 130 of the AWG. Thefree space region then focuses the TE-mode wave and the TM-mode waveonto different image spots, shifted by the birefringence shift inwavelength on the output focal plane. Therefore by positioning theoutput waveguides 150 at these image positions, the polarizationsplitter 100 splits the TE and TM modes for the single channel input. Byvirtue of the FSR, the device will also work for wavelengths that areseparated by integer times of the FSR of the AWG.

FIG. 2 depicts a high level block diagram of an experimental setup forthe polarization splitter 100 of FIG. 1. The experimental setup 200 ofFIG. 2 comprises an unpolarized broad band ASE source (illustratively anErbium doped fiber amplifier (EDFA)) 210, a programmable polarizationcontroller (PPC) 220, an embodiment of a polarization splitter of thepresent invention (illustratively the polarization splitter 100 ofFIG. 1) and an optical detector (illustratively an optical spectrumanalyzer (OSA)) 230. The EDFA 210 is followed by the programmablepolarization controller (PPC) 220. The first stage of the PPC 220 is alinear polarizer. The output of the PPC 220 is connected to a lensedfiber 240 through a connector (not shown). The nominal focal length ofthe lensed fiber 240 is 8 um. The lensed fiber 240 is mounted through afiber holder on a three-axis translation stage (not shown). As such, theoutput of the PPC 220 is optically connected to the polarizationsplitter 100. The output of the polarization splitter 100 is coupled toa cleaved fiber 250, which is also mounted on a three-axis translationstage (not shown) through a fiber holder (not shown). Passbands aremeasured using the OSA 230 at the TM (No.1) output and at the TE (No.5)output of the polarization splitter 100, while the PPC 220 alter theinput state of polarization (SOP) from TM-favored polarization toTE-favored polarization. In the experiment, the transmission ofTE-favored polarization is first maximized at the TE output (No.5) byadjusting the PPC 220 to compensate any polarization changes caused byinterconnecting fiber between the EDFA 210 and the input facet of thepolarization splitter 100.

Subsequently, the first stage of the PPC 220 (e.g., the linearpolarizer) is rotated at a step of six degrees for 90 degrees. Since thetransformation of the SOP by the interconnecting fiber should beunitary, a rotation of the input SOP will cause the same rotation of theSOP at the output of the interconnecting fiber, therefore changing theinput to the polarization splitter 100 from TM-favored polarization toTE-favored polarization.

FIG. 3 graphically depicts the output power of the TM (No.1) output andthe TE (No.5) output of the polarization splitter 100 having an inputwavelength of 1550 nm while changing the input SOP by rotating the inputlinear polarizer of the PPC 220. In FIG. 3, the output power of the TM(No.1) output and the TE (No.5) are plotted versus the relative positionof the PPC 220 in degrees. It can be seen in FIG. 3 that the transmittedlight power swings from the TM (No.1) output to the TE (No.5) output ofthe polarization splitter 100 when changing the input from TM-favoredpolarization to TE-favored polarization. An extinction ratio of 15 dB isachieved for both the TM and the TE modes. Similar polarization beamsplitting is also achieved simultaneously for channels separated fromthe 1550 nm channel by integer numbers of the FSR. For example, FIG. 3further graphically depicts the output power of the TM (No.1) output andthe TE (No.7) output of the polarization splitter 100 for inputwavelengths of substantially 1555 nm (dashed lines) and 1544 nm(dot-dashed lines). Similar performance in terms of extinction ratio isachieved for these channels as well. The limitation of the extinctionratio is mainly from the scattered backgrounds on the focal plane due toimperfect imaging and phase errors in the grating arms of thepolarization splitter 100. These are the same limiting factors for thechannel crosstalk of an AWG.

As depicted in FIG. 3, the maximum fiber-to-fiber transmission power atthe TE (No.5) output of the polarization splitter 100 of FIG. 2 isapproximately 25 dB, while the maximum fiber-to-fiber transmission powerat the TM (No.1) output is approximately −28 dB. The coupling loss forthe cleaved fiber 250 and the lensed fiber 240 are estimated to beapproximately 10.5 dB and 5 dB, respectively. The facets of the fibersare not coated and the reflection loss from the two facets adds about 3dB of loss. Connector losses and the reflections from uncoated fibertips add up to another 1 dB of loss. This indicates that the totalon-chip loss is approximately 5.5 dB for the TE mode and 8.5 dB for theTM mode, assuming that the TM mode and the TE mode have similar couplinglosses. The difference in loss is attributed mainly to the differentpropagation losses of the TE and TM modes. The fact that the TM outputis located at the edge of the Brillouin zone while the TE output islocated near the center of the Brillouin zone may also contributeslightly to the loss difference between the TE output and the TM output.

The polarization splitter of the present invention is wavelengthdependent. In addition, due to the properties of the AWG, thepolarization splitter also performs wavelengthmultiplexing/demultiplexing with the channel filtering given by the AWGfilter passband. The channel filtering can reduce the impact ofbroadband noise such as ASE generated along the transmission link,therefore improve the detected signal-to-noise ratio (SNR). Furthermore,multiple channel operation is achieved since channels that are separatedby the FSR are equivalent in this respect.

The alignment of the output waveguides of the polarization splitter ofthe present invention is very important to the functionality of thepolarization splitter. Furthermore, the simulated effective indexdifference determined above may be different from the actual one,causing an offset between the TE and TM modes. For example, in thepolarization splitter 100, for an input channel at 1550 nm, slight shiftin wavelength of 0.27 nm is observed between the measured TE and TMoutputs. This wavelength shift may cause degradations for data signalspassing through the polarization splitter of the present invention andshould be minimized. For a given waveguide design and layer structure,however, the effective index difference between the TE and TM modes maybe experimentally determined and corrected for in the design of apolarization splitter of the present invention. Because the shiftbetween the TE and TM modes through an AWG is substantially only relatedto the effective index difference and the center wavelength, this errormay be minimized by keeping good repeatability of the polarizationsplitter structure.

In alternate embodiments of the present invention, a polarizationsplitter in accordance with the present invention may split an inputchannel into polarization components other than the TE-mode and TM-modecomponents via the proper placement of the output ports of the AWG.

While the forgoing is directed to various embodiments of the presentinvention, other and further embodiments of the invention may be devisedwithout departing from the basic scope thereof. As such, the appropriatescope of the invention is to be determined according to the claims,which follow.

1. An integrated polarization splitter having a passive portion and anactive portion, comprising: an arrayed waveguide grating (AWG) in thepassive portion, the AWG including: an input coupler; an output coupler;and a plurality of waveguides of unequal length connecting said inputand output couplers; wherein at least two output ports of said AWG arepositioned relative to an input port such that a first polarizationcomponent and a second polarization component of a single channel inputsignal arriving at different phase fronts of a free space region at anoutput side of said AWG are respectively received by separate ones ofsaid output ports such that said first polarization component and saidsecond polarization component are split by said AWG; and wherein thepassive portion and the active portion are integrated in accordance withactive/passive monolithic integration techniques, wherein the activeportion comprises at least one active device for individually modifyingat least one of said first polarization component and said secondpolarization component split by said AWG.
 2. The polarization splitterof claim 1, wherein said first polarization component comprises a TEmode and said second polarization component comprises a TM mode of saidinput signal.
 3. The polarization splitter of claim 1, wherein thepolarization of input signals separated in wavelength from said singlechannel input signal by integer multiples of the free spectral range ofsaid AWG is also split by said AWG.
 4. The polarization splitter ofclaim 1, wherein at least one of said output coupler and said inputcoupler comprises a star coupler.
 5. The polarization splitter of claim1, wherein at least one of said output coupler and said input couplercomprises a slab waveguide lens.
 6. The polarization splitter of claim1, wherein said polarization splitter performs at least one ofwavelength multiplexing and demultiplexing for input signals comprisingmore than a single channel.
 7. The polarization splitter of claim 1,wherein said polarization splitter performs channel filtering.
 8. Thepolarization splitter of claim 1, wherein said polarization splitter isfabricated from optical waveguides, each of said optical waveguidescomprising: a shallow etched burned rib structure passive layer; and athin layer of multi-quantum-wells (MQW) on top of the buried ribstructure functioning as an active layer.
 9. The polarization splitterof claim 1, wherein said polarization splitter further functions as atunable polarization controller.
 10. An integrated polarization splitterhaving a passive portion and an active portion, comprising: an arrayedwaveguide grating (AWG) in the passive portion, the AWG including: atleast one input means for receiving an input signal; a means forcoupling said input signal to said AWG; a means for coupling an outputsignal from said AWG; a plurality of waveguides of unequal lengthconnecting said input coupling means and said output coupling means; andat least two output means; wherein said at least two output means ofsaid AWG are positioned relative to said at least one input means suchthat a first polarization component and a second polarization componentof said input signal arriving at different phase fronts of a free spaceregion of said output coupling means of said AWG are respectivelyreceived by separate ones of said output means such that said firstpolarization component and said second polarization component are splitby said AWG; and wherein the passive portion and the active portion areintegrated in accordance with active/passive monolithic integrationtechniques, wherein the active portion comprises at least one activedevice for individually modifying at least one of said firstpolarization component and said second polarization component split bysaid AWG.
 11. The integrated polarization splitter of claim 10, whereinsaid input signal is a single channel input signal.
 12. The integratedpolarization splitter of claim 10, wherein said first polarizationcomponent comprises a TE mode and said second polarization componentcomprises a TM mode of said input signal.
 13. The integratedpolarization splitter of claim 10, wherein the polarization of inputsignals separated in wavelength from said input signal by integermultiples of the free spectral range of said AWG is also split by saidAWG.
 14. A method of fabricating a polarization splitter having apassive portion and an active portion, comprising: integrating thepassive portion and the active portion using an active/passivemonolithic integration technique, wherein the passive portion comprisesan arrayed waveguide grating, wherein at least two output ports of saidAWG are positioned relative to an input port such that a firstpolarization component and a second polarization component of a singlechannel input signal arriving at different phase fronts of a free spaceregion at an output side of said AWG are respectively received byseparate ones of said output ports such that said first polarizationcomponent and said second polarization component are split by said AWG,wherein said active portion comprises at least one active device formodifying at least one of said first polarization component and saidsecond polarization component.